I'm thinking of changing the IP address of two of my sites from shared to dedicated. I have three sites that will be linked just because it makes sense for them to be linked content-wise. I've read that as far as ranking goes, sites that linked and share the same IP address are discounted. Is this true? Also, when changing IP adresses are there any penalities for established sites?
Changing the IP address for a site is not in itself a problem, and as long as you handle the transition gracefully with little or no downtime there should be no ill-effects.
As for placing related websites on the same server with the same IP address, it is certainly a signal that the sites are related - but in most cases, other elements would already have indicated that they were part of a network (from whois data or toolbar data to the existence of heavy interlinking in the first place). It is impossible to say whether the IP change in itself will tip the balance and alter the way the link relationship is seen by Google or other search engines.
<edited for spelling>
[edited by: encyclo at 12:45 am (utc) on June 9, 2009]
It has been awhile since I checked, but Google actual treats an entire class C (192.168.1.###) as a local neighborhood, so moving to another IP within the same subnet will likely not help with SERPs. Also, ARIN does not permit you using an IP for SEO purposes. They rarely check but should they check you have to justify why you are hosting a site on a dedicated IP address (such as using HTTPS).
I've seen many clients flip IPs and even to different subnets. I think the effectiveness of this on SERPs is limited these days. Whois records, content comparison and crosslinking provide much stronger indications that the sites are closely related.
From my experience. Which is limited, I've only tried it once, recently.
Our site was consistently ranking between 8-11 for a nice keyword on Google.
Switched to a dedicated IP for SSL and other reasons. Since that time the site has dropped for that term to 41-50. That was over 2-3 weeks. Originally we dropped to around 14-16. And I just discovered the big drop recently.
Not sure if it's DC changes or what yet. Since Google is still driving traffic on other keywords.
Lucky for us other traffic is picking up the slack overall, we'll still be down a minor bit for this month at a minimum.